1. Field of the Invention
The present invention relates to a digital filter circuit for use in an analog to digital converter (hereinafter referred to as an "A/D converter") of an oversampling type and, more particularly, to a decimation filter for converting a digital data of a high sampling rate to that of a low sampling rate.
2. Description of the Related Art
An A/D converter of an oversampling type performs A/D conversion at such a high sampling rate as several tens or several hundreds of times of a normal sampling rate. By performing the conversion at such a high rate, it is possible to spread quantization noise to a wide frequency band. By spreading the quantization noise to a wide frequency band, it is possible to make the quantization noise small at a desired frequency band. This means that the signal to noise ratio (i.e., S/N ratio) can be improved by making a rate of sampling faster if the quantization bit number at the A/D conversion is the same. Thus, the advantage is that, by performing the sampling at a high sampling rate for the realization of the same S/N ratio, it is possible to reduce the quantization bit number at the A/D conversion.
When an analog signal is sampled, the signal whose frequency is higher than 1/2 of the sampling frequency (Nyquist frequency) is turned back as an unnecessary signal (turn-back noise) within the Nyquist frequency. It is not possible to eliminate the turn-back noise after the sampling process. Therefore, it is necessary to eliminate the signal whose frequency is higher than the Nyquist frequency before the sampling is conducted. The A/D converter which performs A/D conversion at an ordinary sampling rate has required a very high precision analog filter because the signal frequency band and the Nyquist frequency therefor are substantially the same with each other. However, the A/D conversion circuit of an oversampling type has the advantage that the analog filter to be used can be a comparatively simple one because the signal band and the Nyquist frequency therefor are largely different from each other.
Each of the two advantages mentioned above makes it possible to form the analog circuits simpler so that the A/D conversion circuit of an oversampling type is widely used as a circuit suited to the realization of the semiconductor integrated circuit (the realization of LSIs).
The output data converted by the A/D converter of an oversampling type is a data of a low word (e.g., 1 bit) and of a high sampling rate. This data is excised to a data of a desired sampling rate which is ordinarily used. At this time, if the data of a high sampling rate as it stands is excised and the resultant data is made a data of the desired sampling rate, all of the quantization noise spreading in a wide region turns back into the Nyquist frequency (within the signal band) of the desired sampling rate because the data is processed by a high sampling rate, which causes the input signal to be deteriorated. In order to prevent this, it is necessary that, before the data is excised, the quantization noise distributed outside the signal region be attenuated. A digital filter is used as a means to perform the attenuation but, in doing so, if an attempt is made to obtain the data of the desired sampling rate at one decimation, the digital filter required must have a very high precision. The characteristics required by such filter must unavoidably be that a passing band is narrow, a block band is very wide and a transfer band is very narrow. The realization of filter having such characteristics is technically difficult. Thus, generally, in order to make the characteristics of the digital filter simple, the decimation is performed not only one time but several times. When the decimation is performed several times, the characteristics of the filter for effecting the decimation to an intermediate frequency can be simple because not all the quantization noise is turned back into the signal region so that, generally, a simple filter called a moving average filter is used. It is comparatively easy to realize also a filter for effecting the decimation from the intermediate frequency to a desired frequency as such filter is not extreme in the ratio of the passing band, the blocking band and the transfer band as compared with the filter used when the data of a desired sampling rate is obtained by the one time decimation. In this connection, there is shown in FIG. 1 an example of the related circuit configuration composed of an analog front-end stage (.DELTA..SIGMA.-modulator) F0, a first decimation filter F1 and a second decimation filter F2. An example of the filtering characteristics of the first decimation filter F1 is as shown in FIG. 2(a) and that of the second decimation filter F2 is as shown in FIG. 2(b). FIG. 3 shows a detailed block diagram of the first and second decimation filters F1, F2 shown in FIG. 1. The first decimation filter F1 is formed by a shift register 1, a latch circuit 2 and a decoder 3, and the second decimation filter F2 is formed by a coefficient memory 4, a multiplier 5, an adder 8 and an accumulator 9. A further detailed circuit diagram of the multiplier 5 which is formed by a plurality of gates. full adders represented by a symbol FA and half adders represented by a symbol HA is shown in FIG. 4. Each of the full adders FA has input terminals A, B for receiving an adding value and a value to be added thereto, input and output terminals CI, C0 for a carry signal, and output terminal S for outputting the resultant added value as shown in FIG. 5(a), and each of the half adder HA has input terminals A, B for receiving an adding value and a value to be added thereto, an output terminal C0 for the carry signal, and an output terminal S for outputting the resultant added value as shown in FIG. 5(b).
As above, the object of the circuit of an oversampling type is to convert a digital data of a high sampling rate to a digital data of a desired sampling rate by conducting the decimation several times. In such a conventional circuit, the several decimations are performed independently from one another such that, upon completion of the operation by a first decimation, its output data is merely processed as an input to the next decimation filter so that a problem herein is that the operation circuit concerned inevitably becomes large and complicated.